583. Had a highly-organized and sensitive body been made the instrument of cutting, tearing, and breaking down the food, every tooth, every time it comes in contact with the food, would produce the exquisite pain now occasionally experienced when a tooth is inflamed. Yet a body wholly inorganic and therefore insensible, could not perform the office of the instrument; first, because a dead body cannot be placed in contact with living parts without producing irritation, disease, and consequently pain; and, secondly, because such a body being incapable of any process of nutrition, must speedily be worn away by friction, and there could be no possibility of repairing or of replacing it. The instrument in question, then, must possess hardness, durability, and, to a certain extent, insensibility; yet it must be capable of forming an intimate union with sentient and vital organs, must be capable of becoming a constituent part of the living system. 584. To communicate to it the requisite degree of hardness, the hard substance forming its basis is rendered so much harder than common bone that some physiologists have even doubted whether it be bone, whether it really possess a true organic structure. That there is no ground for such doubt the evidence is complete. For, 1. The tooth, like bone in general, is composed partly of an earthy and partly of an animal substance; the earthy part being completely removable by maceration in an acid, and the animal portion by incineration, the tooth under each process retaining exactly its original form. 2. The root of the tooth is covered externally by periosteum; its internal cavity is lined by a vascular and nervous membrane, and both structures are intimately connected with the substance of the tooth. If these membranes really distribute their blood-vessels and nerves to the substance of the tooth, which there is no reason to doubt, the analogy is identical between the structure of the teeth and that of bone. 3. Though the blood-vessels of the teeth are so minute that they do not, under ordinary circumstances, admit the red particles of the blood, and though no colouring matter hitherto employed in artificial injections has been able, on account of its grossness, to penetrate the dental vessels, yet disease sometimes accomplishes what art is incapable of effecting. In jaundice the bony substance of the teeth is occasionally tinged with a bright yellow colour; and in persons who have perished by a violent death, in whom the circulation has been suddenly arrested, it is of a deep red colour. Moreover, when the dentist files a tooth, no pain is produced until the file reaches the bony substance; but the instant it begins to act upon this part of the tooth, the sensation becomes sufficiently acute. 585. These facts demonstrate that the bony matter of the tooth, though modified to fit the instrument for its office, is still a true and proper organized substance.